1. Field of the Invention
The present invention relates to a resin substrate used for a high density optical recording medium, in which recording and reproduction of information, or recording, reproduction, and deletion of information are conducted by convergence of an optical beam, and more specifically it relates to a substrate for a digital versatile disk that is excellent in moldability and strength characteristics, a resin composition for the substrate, a process for producing the substrate, and a digital versatile disk.
2. Description of the Related Art
As an optical disk for recording and reproduction of information by irradiation with laser light, a magnetoptical disk (MO), a write once optical disk (CD-R etc.), a digital audio disk (compact disk (CD), CD-ROM) and an optical video disk (laser disk) have been known. Among these, a compact disk and a laser disk are read only optical disks, and a write once optical disk and a magnetoptical disk are RAM (random access memory) type optical disks, which are quickly spreading as various recording media.
The standard of a read only digital video disk (DVD-ROM) having a high density recording capacity (4.7 GB) has been established, and since then high density in information recording, high quality in voice recording and high minuteness in image recording of the digital video disk are receiving attention. As a method for recording and reproduction of information for the digital video disk, a pre-pit method is employed for a read only digital video disk (DVD-ROM), a dye recording method is employed for a write once digital video disk (DVD-R), and a phase transition method is employed for a rewritable digital video disk (DVD-RAM) These digital video disks are spreading into not only video but also a wide range of fields of a recording medium for audio and computers, and the standardization thereof has been established or is being considered to be established. The invention relates to a digital versatile disk, which means such a wide range of optical recording media, and the term DVD is used herein as a digital versatile disk.
The information recording density of a DVD is increased by such a manner that the wavelength of the irradiating laser light is shortened from 780 nm to the range of from 635 to 650 nm, and simultaneously the numeral aperture of an object lens is increased from 0.45 to the range of from 0.52 to 0.6, and it is also increased by reducing the thickness of the substrate by half, i.e., from 1.2 mm to 0.6 mm, to shorten the path, in which the laser light travels within the disk substrate.
While a resin, such as a polycarbonate resin, an acrylic resin, and an epoxy resin, and glass are used as a material of a substrate for a DVD, a resin is mainly employed since it can be molded into a precise shape by injection molding, and is suitable for mass production. However, because the thickness of the substrate becomes thinner as reduced from 1.2 mm to 0.6 mm as described above, the impact resistance of the substrate is becoming an important factor. Accordingly, an aromatic polycarbonate resin is used as a material of a substrate in view of strength, such as impact resistance, as well as transparency, heat resistance and low water absorbing property.
The inventors have proposed (1) a digital video disk substrate comprising a polycarbonate-polyorganosiloxane copolymer, or comprising the copolymer and a polycarbonate resin (JP-A-9-265663), (2) a digital video disk substrate comprising a resin having a specific end group and a viscosity average molecular weight of from 10,000 to 17,000 (JP-A-9-320110), and (3) a digital video disk substrate comprising a polycarbonate resin having a viscosity average molecular weight of from 10,000 to 17,000 and a molecular weight distribution (weight average molecular weight (Mw)/number average molecular weight (Mn)) of 2.3 or more (WO97/36292).
There have been also proposed as a substrate suitable for a DVD (4) an optical disk substrate comprising an aromatic polycarbonate resin, in which at least 20% by mol of the total aromatic dihydroxyl component is 1,1-bis (hydroxyphenyl)-3,3,5-trimethylcyclohexane, and various characteristics are specified (JP-A-8-293128), and (5) a substrate for an optical disk comprising a resin composition comprising an aromatic polycarbonate resin comprising from 10 to 60% by mol of a constitutional unit derived from a fluorene-containing aromatic diol and from 90 to 40% by mol of a constitutional unit derived from the other aromatic diol than described above, in which a carbon atom connected to a phenyl group has another phenyl group (JP-A-10-310692).
The substrates (4) and (5) described above uses the extremely special polycarbonate resins, and thus they are inferior in general-purpose properties and are expensive. Furthermore, they relate to a general optical disk, and although there are general descriptions for an optical disk, what is evaluated therein is a substrate having a thickness of 1.2 mm, but a substrate having a thickness of 0.6 mm is not described.
Furthermore, (6) JP-A-9-208684 discloses a polycarbonate resin of a high flowability, which is a polycarbonate resin having a viscosity average molecular weight (Mv) of from 13,000 to 20,000, in which a low molecular weight component having a molecular weight of 1,000 or less contained in the polycarbonate resin is less than 1% by weight, and a polycarbonate oligomer having a molecular weight of from 2,000 to 5,000 is 10% by weight or more. JP-A-9-208684 also discloses that the resin has a good transferability and a high-cycle moldability suitable for producing an optical recording medium, such as a digital video disk, and can reduce attached matters to a stamper. However, there is only a specific evaluation where a substrate having a thickness of 1.2 mm is molded at a resin temperature of 340.degree. C. and a mold temperature of 90.degree. C.
Moreover, (7) JP-A-10-60105 and JP-A-11-35671 disclose a polycarbonate resin composition for an optical disk substrate containing from 0.06 to 0.1% by weight of a fatty acid monoglyceride having from 14 to 30 carbon atoms and from 3.5 to 8% by weight of a low molecular weight component composed of from 1 to 4 carbonate repeating units, and a polycarbonate resin composition for an optical disk substrate containing from 3.5 to 8% by weight of a low molecular weight component composed of from 1 to 4 carbonate repeating units (provided that the cases containing from 0.06 to 0.1% by weight of a fatty acid monoglyceride having from 14 to 30 carbon atoms are excluded).
However, in the case where the addition amount of the fatty acid monoglyceride is 0.06% by weight or more, although the mold release property becomes good, there are problems in optical properties in that fog is formed in the substrate, and there are tendencies on an accelerated deterioration test under a high temperature and a high humidity in that the reduction in molecular weight and formation of polarized white turbid defects due to hydrolysis of the polycarbonate resin are accelerated. This means that the reliability on information recording as a DVD is deteriorated, and it is considered that the resins cannot satisfy practical purposes. There are disclosed evaluation results of the transferability and the mold release property of a stamper on molding in an example where a substrate having a thickness of 0.6 mm is molded at a relatively high temperature condition, in which a temperature of a cylinder of a molding machine is 380.degree. C. and a temperature of a mold is 120.degree. C. However, there is no description about the birefringence and the strength, as well as the molecular weight of the polycarbonate resin.
Furthermore, in the invention of (7), when the content of the fatty acid monoglyceride having from 14 to 30 carbon atoms is less than 0.06% by weight, there are problems in that the mold release property is insufficient, and the high speed moldability and the yield of the product are decreased. Since the polycarbonate resin contained in the compositions contains a relatively large amount of a monomer, there is a possibility of causing a problem in stability in a long period of time. The production process of the resin becomes complicated as comprising separation of the low molecular weight component, drying and re-addition thereof, and thus the production of the polycarbonate having a high purity becomes complicated and highly costly. Furthermore, the publications of (6) and (7) do not disclose influence of an end group of the polycarbonate resins.
While a substrate for a DVD is formed by adhering two substrates each having a diameter of 120 mm and a thickness of 0.6 mm, to realize a strength as a DVD, as well as increase in recording density by recording on both sides, further means for increasing the recording density has been proposed. That is, there is a tendency in that the depth of a pit becomes large from a DVD-ROM to a DVD-R and a DVD-RAM, and in order to transfer and shape the pit, it is necessary to increase the melt flowability, the molding temperature and the temperature of the mold. There are also increasing demands for increase of the moldability and the productivity of the substrate, increase of the yield of the product, and stabilization of the product quality.
In particular, there is a demand for increase of the moldability of the substrate associated with minuteness of the pit shape, particularly solution of both the transferability and the release property. When the release property and the strength of the substrate are insufficient, there is a case in that fine burrs are formed within the mold even though the releasing is apparently conducted without any problem, and the burrs are incorporated into a substrate on molding of the next cycle to become foreign matters causing a defective product, resulting in deterioration in yield of the product.